1. Field of the Invention
The present invention relates to an exhaust system for an outboard motor.
2. Related Art
An internal combustion engines such as gasoline engine discharges an exhaust gas containing harmful components such as carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx). An exhaust gas purifying device for rendering such harmful components harmless is generally classified into two types of devices based on their concepts.
An exhaust gas purifying device of one of these types is a device which induces complete combustion of fuel in a combustion chamber of an engine for preventing generation of harmful components. An exhaust gas purifying device of the other type is device which renders the exhaust gas discharged from a combustion chamber of an engine harmless before the exhaust gas is discharged to air, even if the exhaust gas contains harmful components.
As a system for purifying the exhaust gas of the latter type, there is provided a purifying device in which a catalyst containing, for example, platinum, rhodium and palladium is provided in an exhaust system in a manner such that the catalyst is exposed to the exhaust gas discharged from the combustion chamber of the so as to cause a chemical reaction therebetween for promoting oxidation and reduction of the exhaust gas for changing the harmful components into carbon dioxide (CO2), water (H2O), nitrogen (N2) and the like to render them harmless.
As a catalyst device for an outboard motor, there is provided a device in which the outboard motor includes an exhaust outlet portion in a side surface of an upper portion of an engine, an exhaust gas passage in a lower portion of the engine, a spacer plate secured to cover the exhaust outlet portion and the exhaust gas passage, and a cover mounted to the spacer plate.
Another exhaust gas passage is formed between the spacer plate and the cover, and a catalyst is arranged in this exhaust gas passage.
Further, there is also provided a device in which the outboard motor includes an exhaust gas gathering member secured to a side surface of an upper portion of an engine for gathering exhaust gas discharged from an exhaust port, and a horizontal exhaust pipe and a vertical exhaust pipe which are coupled to the exhaust gas gathering member, and a catalyst is arranged in both the exhaust pipes (refer to Japanese Unexamined Patent Application Publication No. 2000-356123 (Patent Publication 1), for example).
Furthermore, there is also provided a catalyst device for an outboard motor in which an engine of the outboard motor includes exhaust gas passages which are paralleled with one another and are communicated to plural cylinder portions, an exhaust gas gathering portion which is communicated to the exhaust gas passages at the side of the exhaust gas passages opposite from a space, and a catalyst extending from the upper exhaust gas passage to the lower exhaust gas passage, in the exhaust gas gathering portion (refer to Japanese Patent No. 3180118 (Patent Publication 2), for example).
However, the structure which employs the spacer plate requires ensuring of a flat surface for arranging the plate, which causes a catalyst to be arranged far from the engine, resulting in an increasing of the size of the entire outboard motor.
Further, the structure which places plural catalysts in the plural exhaust pipes in different directions requires a complicated structure for holding the catalysts in the exhaust gas gathering member.
Furthermore, the structure which provides a catalyst extending from the upper exhaust gas passage to the lower exhaust gas passage requires a carrier having a cross-sectional shape different from the cross-sectional shapes (round shapes, elliptical shapes and oval shapes) of common carries (base members for catalysts) made of ceramics and metals. Therefore, it is difficult to manufacture such a carrier.